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Ab initio X-ray powder structure determination of parascorodite, Fe(H₂O)₂AsO₄

Natale PERCHIAZZI^1,*, Petr ONDRU² and Roman SKÁLA²

¹ Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, I-56126 Pisa, Italy
² Czech Geological Survey, Klárov 3, CZ-118 21 Prague 1, Czech Republic

^* Corresponding author, e-mail: natale{at}dst.unipi.it

The crystal structure of parascorodite, Fe(H₂O)₂AsO₄, has been solved ab-initio from powder data. The mineral is trigonal, with space group Pc1, Z = 6, a = 8.9232(2) Å, c = 9.9296 (3) Å. The Rietveld refinement was led up to R_p = 4.5 %. The structure is built up by the same coordination polyhedra found in its dimorph scorodite, namely slightly distorted Fe-centred octahedra and As-centred tetrahedra, but their connection lead to quite different crystal structures for the two polymorphs. Parascorodite structure can be described in terms of mixed octahedral-tetrahedral "columns" including Fe2 and As polyhedra, running along c, similar to those found in ferrinatrite and kaatialaite. These columns are interconnected in a threedimensional framework by the sharing of the 01 oxygen between [AsO4] tetrahedra and Fe 1 octahedra, and by the strong hydrogen bond OW...O2.

Key-words: parascorodite, Kak, powder data, ab-initio, structural solution.

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